Alloyed gray cast iron

ABSTRACT

A TOUGH GRAY CAST IRON HAVING EXCELLENT TENSILE STRENGTH AND IMPACT MODULUS, CONTAINING APPROXIMATELY 2.5 TO 3.8% CARBON, LESS THAN THAN 0.08%, AND PREFERABLY LESS THAN 0.04% PHOSPHORUS, LESS THAN 0.08% AND PREFERABLY LESS LESS THAN 0.06%, AND PREFERABLY LESS THAN 0.3% SULPHUR, A CONTENT OF MANGANESE GREATER THAN 0.3% AND UP TO 2.25%, PREFERABLY GREATER THAN 0.3% AND UP TO 1.75%, THE BALANCE BEING IRON WITH LOWEST CONTENT OF DISTURBING ELEMENTS AS WELL AS A SILICON CONTENT WHICH IS REDUCED BY ABOUT 0.8% ABSOLUTELY TO 1/2 OF THE SILICON CONTENT REQUIRED IN CAST PIECES HAVING THE SAME WALL THICKNESS MADE OF CONVENTIONAL GRAY CAST IRON.

United States Patent US. Cl. 75-126A 13 Claims ABSTRACT OF THE DISCLOSURE A tough gray cast iron having excellent tensile strength and impact modulus, containing approximately 2.5 to 3.8% carbon, less than 0.08%, and preferably less than 0.04% phosphorus, less than 0.08%, and preferably less than 0.04% chromium, less than 3 ml./100 g. hydrogen, less than 0.06%, and preferably less than 0.03% sulphur, a content of manganese greater than 0.3% and up to 2.25%, preferably greater than 0.3% and up to 1.75%, the balance being iron with lowest content of disturbing elements as 'well as a silicon content which is reduced by about 0.8 absolutely to /2 of the silicon content required in cast pieces having the same wall thickness made of conventional gray cast iron.

This invention relates to tough gray cast iron and particularly to tough gray cast iron having excellent tensile strength and impact modulus with higher manganese contents than heretofore believed acceptable.

U.S. Letters Patent No. 3,375,103, assigned to the same assignee as the instant application the disclosure of which is incorporated herein in its entirety by reference, describes the use of an alloyed gray cast iron having lamellar graphite formation and a largely undistorted lattice of the base iron mass or matrix as working material for cast pieces possessing great toughness and high heat conductivity when in cast condition. This cast iron is characterized by particularly low contents of carbide forming and carbide-stabilizing iron associates, and it is composed of 2.5% to 3.8% carbon, less than 0.08% phosphorus, less than 0.08% chromium, less than 3 ml./l00 g. hydrogen, less than 0.06% sulphur, less than 0.3% manganese, the balance being iron with lowest content of disturbing elements, as well as a content of silicon which is reduced by about 0.8% to /2 of the silicon content required in cast pieces having like Wall thickness made of conventional gray cast iron. It has now been determined that the invention described in the aforesaid U.S. Letters Patent is also applicable for a type of alloyed cast iron which shows a higher content of manganese by retaining the same purity of the base mass.

Accordingly, the instant invention is directed to an alloyed gray cast iron for use as a workpiece for east pieces having lamellar graphite formation, great toughness and high heat conductivity in cast condition, consist ing essentially of:

(a) 2.5% to 3.8% carbon;

(b) less than 0.08% phosphorus, preferably less than 0.04% phosphorus;

(0) less than 0.08%, preferably less than 0.04% chromiurn;

(d) less than 3 ml./ 100 g. hydrogen;

(e) less than 0.06%, preferably less than 0.03% sulphur;

(f) the balance being iron with lowest content of disturbing elements, as 'well as a silicon content which is reduced by about 0.8% absolutely to of the silicon rod content required in cast pieces having the same Wall thickness made of conventional gray cast iron;

with the provision that the content of manganese is greater than 0.3% and up to 2.25%, preferably greater than 0.3% and up to 1.75% manganese. It was further determined that as the sulphur content becomes smaller within the range which is the subject of the instant application, the toughness increases with the manganese content being retained constant.

Compositions which were found to be particularly useful for this type of alloyed cast iron having lamellar graphite formation were such compositions which possess a manganese content of more than 0.3% and up to 1%, having high purity of the matrix. Cast pieces made of alloyed cast iron having manganese content within this range possess high mechanical properties concerning toughness and impact modulus. Especially favorable properties concerning toughness and impact modulus of cast pieces made from the inventive alloyed cast iron are obtained if the content of manganese is greater than 0.3% and up to 0.6%.

In order to obtain optimum toughness properties, the ratio of manganese to sulphur in the alloyed cast iron of the invention should be greater than 5 to 1. In general, this ratio is preferably greater than 10 to 1, but for practical purposes will not exceed 300 to l.

The alloyed cast iron of this. invention possesses the good technical casting properties of normal gray cast iron and enables casting of pieces with attendant lamellar graphite formation having high values for toughness and elongation. Morevoer, it is possible to produce workpieces having high heat conductivity and increased toughness as compared with cast pieces made of normal gray cast iron. In addition thereto, the inventive alloyed cast iron enables production of work pieces having high wearability and improved toughness. The designation toughness (T) can be expressed as follows;

In this equation a stands for impact modulus, measured in kp. cm./cm. and 6 stands for tensile strength measured in kp./mm? The silicon content of the cast piece made from the alloyed cast iron of this invention is remarkably low and is considerably reduced as compared with cast pieces of the same wall thickness made from conventional gray cast iron. Thus, in contrast with a cast piece made of conventional gray cast iron with a value of up to 1.6% silicon, a cast piece formed of tough gray cast iron may be cast having a silicon value which is reduced by at least one half. If, for example, a cast piece is being cast from conventional gray cast iron having 1.5% silicon, then as compared therewith, if the same cast piece is being cast from tough gray cast iron, at most only half of this value is required, i.e. 0.75% silicon.

Cast pieces of an alloyed cast iron which would otherwise have a silicon content of above 1.6%, if cast from tough gray cast iron would have the silicon content reduced by at least 0.8% absolutely. For example, if a cast piece is being cast from conventional gray cast iron with 2.5% silicon, then the identical cast piece from tough gray cast iron is cast with no more than 1.7% silicon.

The person skilled in the art has no difliculty in determining in each individual case the absolute silicon content of the cast piece made from conventional gray cast iron. It must be considered here that this content, as is well known, is also dependent upon the wall thickness of the cast piece. This is the basis upon which, in the individual case, the silicon content must be determined as far as tough gray cast iron is concerned.

It is of advantage to perform an inoculation which is known in connection with gray cast iron for the inventive alloyed cast iron metal in order to achieve the gray solidification. The only thing that has to be observed in this connection is that the alloying inoculant which is generally formed on a silicon basis, introduces a certain silicon content into the cast iron melt. This silicon content which is being introduced into the melt through the inoculant must be considered in determining the final silicon content or value; and it is, therefore, necesesary to provide for a correspondingly lower silicon content of the base melt.

It is especially important to consider this fact under conditions where the silicon values of the cast iron melt are extremely small.

Concerning the various alloying possibilities of the inventive alloyed cast iron, the explanations set forth in the above-mentioned U.S. Letters Patent are similarly valid. This means that the alloy element must possess none or only a very weak carbide-stabilizing effect, it must possess a lattice constant resembling that of iron in order to largely avoid a distortion of the ferrite lattice and lastly, the alloy element should not combine with carbon.

The alloyed cast iron according to this invention can be produced by metallurgical processes which are known as such and are also mentioned in the aforesaid U.S. Letters Patent.

As was pointed out of the aforementioned U.S. Letters Patent, it is generally known and believed that an increaes of the manganese content has the tendency of impairing the plastic deformability of gray cast iron and the presence of the fanganese in the ferrite lattice causes a distortion. However, it has been found according to this invention that a tough gray cast iron having excellent tensile strength and impact modulus can be provided with increased manganese content so long as the other constituents of the composition are maintained within the specified ranges.

Therefore, it is a primary object of this invention to provide a tough gray cast iron of excellent tensile strength and impact modulus having contents of manganese greater than 0.3% and up to 2.25%, preferably greater than 0.3% and up to 1.75%.

Another object of this invention is to provide a gray cast iron of improved properties including good castability.

Other and further objects will be obvious from the following detailed description and the instant invention concepts will be beter understood by reference to the :1 cast iron of the following final analysis was cast into 30 mm. test bars from which were removed test pieces to determine tensile strength (short proportional bar according to DIN 50125 B) and impact modulus (according to ISO-Proposal No. 1197 Percent C 3.03 Si 1.31 Mn P 0.022

1 See Table I.

TABLE I Percent a (k.p./ a (cm.

Mn mm?) k.p./cm. T

The above Table I depicts the toughness values of tough gray cast iron expressed by toughness T with increasing manganese contents and otherwise unchanged analysis.

In order to achieve, in conventional or normal gray cast iron, tensile strength values of 38 to 40 kp./mm. one would have to cast test bars from a cast iron of the following composition:

Percent C 2.92 Si 2.15 Mn 0.72 P 0.110 S 0.095 Cr 0.2

In a cast iron of the immediately above composition, however, the impact modulus amounts only to 8.5 cm. kp./cm. For a tensile strength of 38 kp./mm. there results a value of 1.03 for the toughness T.

EXAMPLE 2 After inoculation with the commercial inoculant of Example 1, a cast iron of the following final analysis was cast into 30 mm. test bars, and from tehse test bars test pieces were removed in order to determine tensile strength and impact modulus according to Example 1.

Percent C 2.95 S1 0.96 Mn 0.32 P 0.032 S Cr 0.01

1 See Table 11.

TABLE 11 Percent Mn Percent S mmgj 011 1 5) T 0. 32 O. 058 39. 0 13. 5 l. 15 0. 32 0. 043 40. 0 16. 0 1. 0. 32 O. 020 40. 1 l8. 0 1. 220 0. 32 0. 009 38. 5 18. 5 l. 240

From the values listed in the above Table II there can be observed the increase of the toughness at constant manganese content, but reducing sulphur content.

EXAMPLES 3-7 In each instance 1350 kg. cast iron with an analysis listed in Table III were melted in a 1.5 t. NFI (low frequency induction) furnace and tapped at 1490 C. The cast iron melt was then cast into cast pieces of running production, such as thin walled housing parts. The inoculation of the individual cast iron melts occurred in each instance during tapping from the furnace by adding a commercial inoculant consisting of Percent Silicon 60-65 Manganese 5-7 Zirconium 5-7 Aluminum 1 Iron Balance Then tests were taken with the aid of 30 mm. test bars which had been cast together with the cast pieces. Such tests concerned the tensile strength (short proportional bar according to DIN 50125 B) and the impact modulus (according to ISO-Proposal No. 1197). Table III shows the mechanical properties for the corresponding composition of the alloyed cast iron.

TABLE III 5 Percent 1 The examples assembled in Table III show that cast 5 pieces having great toughness can be produced from cast iron melts having high purity and with manganese contents which are greater than 0.3% and up to 1% manganese as well as sulphur contents which are smaller than 0.06%. It is to be observed that the manganese-sulphur ratio of the inventive alloyed cast irons is larger than to 1.

In contrast thereto, tests with comparable test bars formed of conventional gray cast iron having carbon contents of 3.6 to 3.8% resulted in substantially lower values for the impact modulus which is characteristic for the toughness of cast iron with lamellar graphite.

Table IV lists the values derived from test bars of conventional gray cast iron. The 30 mm. test bars were cast under the same conditions as those in Examples 37 and then test pieces were prepared in order to determine the tensile strength and the impact modulus.

(b) less than 0.08% phosphorus;

(0) less than 0.08% chromium;

(d) less than 3 ml./100 g. hydrogen;

(e) less than 0.06% sulphur;

(f) a manganese content greater than 0.3% and up (g) the balance being iron with lowest content of disturbing elements, as well as a silicon content which is reduced by about 0.8% absolutely up to /2 of the silicon content required. for conventional gray cast iron of cast pieces with the same wall thickness.

2. An alloyed cast iron according to claim 1, wherein said manganese content is greater than 0.3% and up to 1%.

3. An alloyed cast iron according to claim 1, wherein the ratio of manganese to sulphur is greater than 5 to 1.

4. An alloyed cast iron according to claim 2, wherein the ratio of manganese to sulphur is greater than 5 to 1.

5. An alloyed cast iron according to claim 1, wherein said manganese content is greater than 0.3% and preferably up to 0.6%

6. An alloyed cast iron according to claim 1, wherein the ratio of manganese to sulphur is preferably greater than 10 to 1.

7. An alloyed cast iron according to claim 1, wherein the ratio of manganese to sulphur amounts to as much as 300 to 1.

8. An alloyed cast iron according to claim 2, wherein the ratio of manganese to sulphur is preferably greater than 10 to 1.

9. An alloyed cast iron according to claim 2, wherein the ratio of manganese to sulphur amounts to as much as 360 to 1.

10. An alloyed cast iron as defined in claim 1, containing preferably less than 0.04% phosphorus.

11. An alloyed cast iron according to claim 1, containing preferably less than 0.04% chromium.

12. An alloyed cast iron according to claim 1, containing preferably less than 0.03% sulphur.

13. An alloyed cast iron according to claim 1, wherein the manganese content is preferably greater than 0.3% and contains up to 1.75% manganese.

References Cited UNITED STATES PATENTS 1,746,467 2/1930 Greiner 16555X 1,826,211 10/1931 Greiner 164---55X 2,352,408 6/1944 Reece 148-3 3,375,103 3/1968 Collaud -l23 3,299,482 1/ 1967 Tache 75--130X References Cited FOREIGN PATENTS 1,005,450 9/1965 Great Britain 75-123CB 1,094,856 12/1964 Great Britain 75l23OB 1,176,583 1/1970 Great Britain 75--123CB L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. XJR.

75'123CB, 126B, 126J, 126K, 126L, 126Q 

